Genetic origin of Mesozoic natural gases in the Ordos Basin (China): Comparison of carbon and hydrogen isotopes and pyrolytic results

Permian coal and Triassic mudstone from the Ordos Basin were pyrolyzed in a closed system using a gold tube technique. Carbon and hydrogen isotopes of the gases generated from pyrolysis were compared to Mesozoic gases in the basin to interpret the origin, maturity and any mixing of gases. Maturation trends for thermogenic methane from both coal and lacustrine kerogens in our experiment were found to be independent of heating rate, allowing their use for determination of gas provenance. Gases from a tectonically stable area like the Shanbei slope are derived mainly from Yanchang lacustrine kerogen, and gases in tectonically active areas consist of mixtures of coal-derived gases and oil-associated methane from deeply buried formations, as well as oil-associated gases and biogenic gases from shallow depth. The thermal maturity of the C2 and C3 gases is estimated to cover an equivalent vitrinite reflectance range from 0.7% to 1.2% Ro, whereas C1 gas exhibits a wide maturity variation ranging from 0.5% to 1.5% Ro and implying significant mixing of Mesozoic methane in the basin.